New Mexico Geological Society Annual Spring Meeting — Abstracts

The Kinney quarry exposes a Virgilian sequence in the Madera Group that changes up-section from a dark gray, hard, platy, argillaceous limestone containing a restricted marine fauna, to brown/olive/gray delta-plain shales and siltstones characterized by plants and a variety of brackish to fresh water invertebrates and vertebrates. The basal limestone, which forms the quarry floor, was sampled randomly for fossils across about 400 square meters, through a stratigraphic thickness of about 30 cm. Nearly 1,000 specimens were collected. The limestone fauna is dominated by the infaunal inarticulate brachiopod Lingula cf. L. carbonaria (21% of total specimens), two specimens of the infaunal bivalve Solemya (S. radiata - 18% and S. trapezoids - 6%), and the epifaunal bivalve Myalina aff. M. wyomingensis (14%). Important subsidiary members of this assemblage are conchostracans and smooth ostracods (each 8%), the pectinid bivalve Dunbarella striata (6%), small unidentified ammonoids (6%), and the brachiopod Chonetinella flemingi (5%). Other bivalves and brachiopods, gastropods, and orthocerid nautiloids are uncommon (each less than 2%), and bryozoan and crinoid are very rare. Disarticulated skeletal remains of fish and plant fragments are moderately common but were not included in the abundance calculations.

The generally unbroken state of the invertebrate skeletons, preservation of spines on Chonetinella, and relatively high percentage of articulated bivalve and Lingula shells (typically 20-25%, ranging to 75% for S. trapezoides) strongly suggest quiet water conditions. The dominant constituents of this assemblage (Lingula, Solemya, Myalina) have been reported often associated in shallow marine, nearshore environments in Midcontinent and eastern U. S. Pennsylvanian sequences; the first two genera have also been interpreted as being locally tolerant of reduced salinity. Chonetinella, and the rarer articulate brachiopods (e.g., Antiquatonia, Composita), bryozoans and crinoids are common in normal-marine shallow limestones lower in the Madera Group; however, the Kinney limestone environment was generally not favorable for them. Ammonoids and nautiloids were stenohaline inhabitants of the open ocean; they probably floated into the Kinney environment and died there, rather than being permanant inhabitants. Late Paleozoic conchostracans and smooth ostracods typically inhabited non-marine, brackish conditions; their occurrence in the limestone unit is best explained by transportation from onshore bodies of water. Terrestrial plant fragments and organic debris were also derived from nearby land areas.

Taken together, the evident is consistent with the interpretation of the Kinney limestone unit as representing a sheltered, quiet, shallow bay or lagoon, connected sufficiently with more open marine waters to maintain salinity and carbonate content at normal or near-normal marine levels. This lagoon was subject, however, to the influx of fresh water, muddy siliciclastic sediments, organic detritus, and some organisms from the surrounding, vegetated, terrestrial environments.